18.3.3 Customizing Saving of Files

If the value of the variable require-final-newline is
t, saving or writing a file silently puts a newline at the end
if there isn't already one there. If the value is visit, Emacs
adds a newline at the end of any file that doesn't have one, just
after it visits the file. (This marks the buffer as modified, and you
can undo it.) If the value is visit-save, Emacs adds such
newlines both on visiting and on saving. If the value is nil,
Emacs leaves the end of the file unchanged; any other non-nil
value means Emacs asks you whether to add a newline. The default is
nil.

Some major modes are designed for specific kinds of files that are
always supposed to end in newlines. Such major modes set the variable
require-final-newline to the value of
mode-require-final-newline, which defaults to t. By
setting the latter variable, you can control how these modes handle
final newlines.

Normally, when a program writes a file, the operating system briefly
caches the file's data in main memory before committing the data to
disk. This can greatly improve performance; for example, when running
on laptops, it can avoid a disk spin-up each time a file is written.
However, it risks data loss if the operating system crashes before
committing the cache to disk.

To lessen this risk, Emacs can invoke the fsync system call
after saving a file. Using fsync does not eliminate the risk
of data loss, partly because many systems do not implement
fsync properly, and partly because Emacs's file-saving
procedure typically relies also on directory updates that might not
survive a crash even if fsync works properly.

The write-region-inhibit-fsync variable controls whether
Emacs invokes fsync after saving a file. The variable's
default value is nil when Emacs is interactive, and t
when Emacs runs in batch mode (see Batch Mode).